text:
Why are trailing edge flaps used for landing?
Why are flaps used when landing? Doesn’t a flap increase lift?
Edited: why are there so many different flaps on the back of a wing? Isn’t one just used when taking off and landing? Are the others backup?

What do you mean by "so many different flaps"?  Do you mean so many "flap surfaces" on a given aircraft or do you mean so many flap designs across all aircraft?

Does this answer your question? Why and when to use flaps?. Plus many other answers about flaps...

@jwh20, I think they mean flap types per this:  https://aviation.stackexchange.com/questions/98539/which-one-of-these-flaps-is-used-on-take-off-and-land

I suspect "so many different flaps" is referring to the different kinds of moveable surfaces on the trailing edge of the wing, including flaps, ailerons, and spoilers.

Yeah, Fred, that explains my question well

Flaps increase the stalling lift coefficient of a wing. They can also change the angle of attack for a given lift coefficient.
By allowing the wing to operate at a higher lift coefficient without stalling, the airplane is allowed to fly slower (without stalling).
$C_L = \frac{W}{0.5\,\rho\, V^2\, S_\mathrm{ref}}$ can be rearranged and made specific to stall:
$V_\mathrm{stall} = \sqrt{ \frac{W}{0.5\, \rho\, S_\mathrm{ref}\, C_{L,\mathrm{max}}}}$
So, a wing that previously stalled at CL of 0.8 might instead stall at a CL of 1.2 with flaps. This is a 50% increase in CL -- which will reduce Vstall to 81% of the un-flapped value.
By deploying flaps for landing, an aircraft can fly slower.  This makes for a safer landing and also for shorter runways.
Flaps also increase drag -- which makes it easier for an airplane to slow down.  So, the landing configuration of flaps is usually the maximum possible -- increase lift as much as possible and increase drag while you're at it.
In contrast, during takeoff, you want the reduction in takeoff speed -- but you don't want a bunch of extra drag.  Consequently, most aircraft use a moderate setting of flaps for takeoff.
Flaps can be deployed fully or partially. Most flaps rotate. Some rotate and translate -- these are called Fowler flaps. Even for flaps that have combined translation and rotation, the pilot's controls are still marked just in terms of the angle. So, you might deploy flaps 10, 20, or 30 degrees. A takeoff setting might be 20 degrees with a landing setting 30 degrees.
If an airplane has multiple spanwise flaps installed, you will use them all at once, they are not just redundancy, they give a full capability.  You want to reduce the landing speed as much as possible.
If you're wondering why a particular aircraft has many flap segments across the wingspan?  This is because wings are actually quite flexible.  The flap is also flexible -- but not exactly the same flexibility as the rest of the wing.  Consequently, when the wing bends, the flap also bends, but not the same amount.  In order to keep the mechanism (hinge, track, etc) from binding, control surface segments are kept short.  That way, the deflection difference over any segment can be accounted for, but it is not as large as it would be for a full-span control surface.
If that doesn't answer your question, please clarify and we'll try some more.

Hi, rob how can they fly slower with the flap deflected please? I don’t really understand that bit? Also, edited my question , sorry I wasn’t really clear first time

@FlightWatcher You need airspeed + wings to get sufficient lift to fly. Flaps effectively change the shape of the wing, in such a way that the lift is increased. So you need less airspeed to keep flying.

@FlightWatcher Lift is function of airspeed, angle of attack and the wing shape+area. More speed = more lift. Higher AoA = more lift (until the airflow separates and you stall, losing almost all lift). More wing = more lift. At all times lift should approximately equal weight of the plane (plus or minus a bit if you want to gain or lose altitude). Less speed = less lift. To maintain lift while slowing down to landing we can adjust the AoA or adjust the geometry of the wing. Flaps allow boosting the lift without going to excessive AoA as the plane slows down to a safe landing speed.

The difficult part of landing an aircraft isn't losing height — it's losing speed.  Losing height without losing speed is called falling (or diving, when done deliberately) and coming into contact with the ground while falling at high speed is, in layman's terms, commonly described as crashing.
Specifically, to land an aircraft, you want to lose as much speed as possible while the aircraft can still maintain horizontal flight and then gently touch the runway and brake to lose any remaining horizontal speed ASAP before you run out of runway.  In fact, if it was possible, the ideal landing would be one where you'd come to a full stop in the air before even touching the ground and then just gently float straight down to land.
Unfortunately only helicopters (and a very small number of other VTOL aircraft like the V-22 Osprey) can actually pull off a landing like that.  The rest, including most fixed-wing airplanes, have a minimum forward speed that they need to maintain in order for their wings to generate enough lift to counter their weight and maintain level horizontal flight.
Flaps divert the air flowing around the wings downwards and thereby generate extra lift, which allows the aircraft to fly slower and still maintain horizontal flight.  Conveniently (for landing) this also causes flaps to generate extra drag, which helps slow down the aircraft.  That, in simple terms, is why flaps are used for landing.
(The reason flaps are not used all the time is that during high speed cruising flight the extra lift is unnecessary and the extra drag is unwanted.  So the flaps are retracted during cruise to reduce drag, which allows the plane to fly faster while using less power from the engines — and thus less fuel — to maintain its speed.)

The questioner's confusion is understandable, because pilots often loosely use the word "lift" in a way that is not really accurate.
In the rigorous sense of the word-- an actual force, measurable in pounds or Newtons-- lift does not vary nearly as much as one might think.  In linear flight, lift must equal weight * cosine (climb or descent angle), so for shallow climbs and glide angles, lift must be nearly equal to weight.  (And note that lift is actually less in a steep climb than in horizontal flight.)
Lift is proportional to lift coefficient times airspeed squared.  When we talk about "high lift" devices, what we are really talking about is a way to increase the lift coefficent, which means that for any given airspeed, the lift force will be increased.  This also means that for any given required lift force, airspeed is decreased.  That's the key to understanding why we use flaps for landing-- we can fly slower without stalling.
An additional benefit of flaps is that they decrease the ratio of lift coefficient to drag coefficient, which is also the ratio of Lift to Drag.  The glide angle is equal to the arctan ((D-T) / L), where T is thrust, or if thrust is zero, the glide angle is simply the arctan (D / L).  So decreasing the L / D ratio makes the glide path steeper (for any given Thrust setting), which makes it easier to judge the final approach.  With jet engines that have some delay in powering up, there's an advantage in being able to operate at a higher power setting for any given descent path, and decreasing the L / D ratio facilitates this.
For take-off, the situation is different-- we want to be able to support the plane's weight at the lowest possible airspeed, so we want a high lift coefficient, but we also want to keep Drag as low as possible, because we need to accelerate throughout the takeoff run, and in many cases we also want to continue accelerating well into the climb.  (Also, in the case where the airspeed is constant so all the excess thrust is used for climbing rather than accelerating, the climb angle that we can achieve is equal to arctan ((T-D) / L).)  So we have a trade-off -- we want to increase the lift coefficient, but without increasing the drag coefficient too much.  Therefore we'll use a more limited deflection of the various different flaps than we would for landing.
Some of the moving surfaces that you see on wings are called "spoilers".  Spoilers decrease the lift coefficient and increase the drag coefficient.  On a large airliner where minimizing the landing speed is important, spoilers would not be deployed until the wheels touch the ground.  On the other hand, sailplanes (gliders) will often deploy spoilers (also called "dive brakes") throughout the final approach and landing because the slight increase in touchdown speed is not really a problem, and the steeper descent path is very beneficial-- with spoilers closed, the final approach would be so flat that it would be very difficult to accurately hit the desired touchdown spot.  Yet some sailplanes use flaps rather than spoilers to steepen the glide path, just like most powered airplanes.  And some use both.
No aircraft is designed with extra moving surfaces "just for backup", at least in the civilian world.  Each part of the flap system is there for a specific reason and will be used whenever that will help optimize the aircraft for the task at hand.  Note that especially with lighter aircraft, different amounts of flap deployment are optimal for different tasks, even in the realm of approach and landing-- e.g. short field landings, soft field landings, landings in strong gusty crosswinds, etc.  And likewise for takeoff -- a different flap setting would often be selected for a short, soft field with no obstacles at the far end, than for a paved runway with tall trees or hills at the far end.
Note that with multi-engine jet aircraft with wing-mounted engines, the flaps are often split into different sections simply to avoid the jet wash.  In such a case the pilot does not have the option to deploy only the inboard flaps, or to deploy only the outboard flaps.  (Another answer has addressed this in more detail-- avoiding the jet wash is not the only reason for splitting up flaps into segments.)
Some related questions and answers--
Does lift equal weight in a climb? (see various answers)
Is excess lift or excess power needed for a climb?
Why is the L/D ratio numerically equal to the glide ratio?  (see various answers)

Re "On a large airliner where minimizing the landing speed is important, spoilers would not be deployed until the wheels touch the ground."  -- not sure why I said that, I guess I meant they would not likely be open during the landing flare.  They are sometimes open in flight-- see for example https://aviation.stackexchange.com/q/47945/34686 .


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How is RNP's onboard performance monitoring and alerting system represented in the cockpit?
I'm trying to understand what is exactly onboard performance monitoring and alerting system related to RNP. Does it display some extent of deviation or just beeps as alarm?
And if that system exists only in RNP, so how can pilot be sure that he performs RNAV1 (for example) correctly?

I'd have to dig, and it may be aircraft specific, but typically there is numerical value displayed in a corner of the PFD when you are in that nav mode indicating "confidence factor" or some such label that must meet a certain criteria for the planned approach.  I will delete this once a better answer is given, just a quick response that might satisfy some curiosity...

In Airbuses logic is exactly like described by @RalphJ on 737.
REQUIRED navigation performance (RNP) is determined by flight management system based on phase of flight or type of approach being flown. Pilot can override this value if required.
Actual navigation performance is calculated based on available sensors and displayed as ESTIMATED position uncertainty (EPU).
Both values are displayed on the bottom row of MCDU PROG page. EPU is required to be lower than RNP, which is indicated by “HIGH” navigation accuracy between the values. If EPU becomes higher than RNP an amber “NAV ACCUR DOWNGRAD” is displayed on both navigation display and MCDU scratch pad, as well as “LOW” accuracy on MCDU PROG page.
GPS PRIMARY indicates valid GPS position. If that is lost, a “GPS PRIMARY LOST” is indicated in similar fashion. This may or may not be relevant as some procedures and airspaces require GPS while others don’t.
Also, if loss of navigation accuracy is due to loss of equipment the failure also generates ECAM messages.

Thank you! But what's about general RANV? How to determine correct performance on it?

RNAV is an old term that dates years before any of the modern systems were in use. Basically “as it is”, it means only that you are able to fly between two arbitrary points. But these days terminology is very confusing where terms RNAV and RNP are used interchangeably and airspace mentioned as RNAV may actually require monitoring capabilities provided by RNP. And even then, different classes (RNAV/RNP1, RNAV5, RNAV10, RNAV4 etc) are not sub/supersets of each other…
See https://aviation.stackexchange.com/questions/51112/what-is-the-difference-between-rnav-and-rnp

In Airbus context there is a specific procedure that if your navigation accuracy is LOW (ie EPU greater than RNP) and airspace and/or procedure does not require alerting you can continue to use RNAV functions of the flight management system after cross checking FMS position with VOR/DME. If the difference is “acceptable”, you are good to go. But of course, you can’t fly any RNP procedures with it.

On the 737 NG and Max the ANP (actual navigation performance) and the RNP (required navigation performance) are shown at the bottom center of the Navigation Display:

Source: http://www.b737.org.uk/images/fltinsts_max-nd-map-full.jpg
If the ANP becomes larger than the RNP, the message "UNABLE RNP" appears in the CDU scratchpad as well, and the RNP/ANP values turn amber on the screen.
It's possible to see the values in FMC pages as well, but on-screen on the ND is the easiest.
There is always some value for RNP, which the FMC computes based on the phase of flight (departure, enroute, arrival, approach) and if the pilot has entered one (which then takes priority).  The message will appear if the ANP exceeds RNP.  That's most likely on approach when RNP values are small, but in a situation when GPS isn't available, the ANP will grow over time and you could be in an "unable RNP" situation in an enroute or arrival scenario as well.

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General rule for CG shift margins and possible databases for existing aircraft
I am currently working on a weight and balance problem focused mostly on loading operations, and I want to add a new feature that compares the calculated CG travel and those of existing aircraft. Do any of you know where I may find such information or if there is a general rule depending on design and aircraft type?
I already looked in Jane's and no such information is available and aircraft flight manuals usually include only the CG limits, not its average excursion.
Thank you!

Are you talking about the calculated CG travel in flight due to fuel burn? Or possibly the the typical standard deviation of loads for an aircraft compared to loads of other aircraft types? Or perhaps the %MAC length from forward limit to aft limit of a given aircraft to other aircraft?

It isn't clear what you mean by "average excursion"...  Nobody keeps records of an aircraft's loaded CG beyond the end of the day.  If a form is created showing the calculation, it is discarded some time after the plane lands safely.  As long as there is no crash and the CG is in limit, the actual CG data for that flight is lost to the sands of time.  What actual "problem" are you working on?

"... that compares the calculated CG travel and those of existing aircraft." this sentence makes no sense.

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Why are both ATC and TCAS tasked to prevent collisions although TCAS takes precedence anyway?

« The primary purpose of ATC worldwide is to prevent collisions
  [...] Many aircraft also have collision avoidance systems, which
  provide additional safety by warning pilots when other aircraft get
  too close. » (source)

Why is ATC tasked to prevent collisions, when for this specific purpose aircraft already have "collision avoidance systems" on board, which even take precedence over ATC in case of conflict ?
This may look like an apparent contradiction to laymen.
I presume that there are several reasons for this, including:

Because not all aircraft are TCAS-equipped.
Because ATC ensures separation minima (although TCAS could be extended to enforce separation).

Which other reasons are there ?

TCAS will give quick results while ATC results will be delayed. I don't see any logic in external system taking the preference when reliable aircraft internal system is available.

How is this question substantially different from the other one you just asked?

TCAS is sort of a fallback, last-resort system used in case all else fails. TCAS works within the scope of what happens within the next minute or so, and will require the involved aircraft to perform fairly extreme maneuvers to avoid a collision. TCAS has one single purpose: to ensure that two (or more) aircraft that have already gotten much too close will not collide. TCAS responds to a single, imminent danger that should not normally exist.
In contrast, ATC is a hugely complicated system that keeps planes moving safely and efficiently through the complex network that is our air infrastructure. Air traffic is carefully planned by ATC up to several hours in advance, and clearances and instructions issued by ATC ensure that planes are kept well clear of each other, so that close encounters do not happen at all.
As an analogy to road traffic, you can think of ATC as a set of traffic lights controlling an intersection, by taking input from a number of sensors and showing red/amber/green lights. TCAS on the other hand, is more comparable to the emergency auto-brake systems that many modern cars have, that will stop the car if, for example, a pedestrian suddenly crosses the road in front of the car.
So why does TCAS take precedence? Because in order for the TCAS to have been triggered at all, something else has already gone wrong - the main system has somehow failed, so the fallback system steps into effect. Looking at our road analogy, suppose you have driven into in intersection because you have a green light. If a car suddenly appears in front of you, you want the emergency brake to kick in, even if the light is indeed green and you should be allowed to go.

While ATC's primary goal is to prevent collisions, it is also responsible for the efficient movement of aircraft through the airspace. That requires a strategic approach to managing air traffic to get aircraft from point A to point B safely and in a timely manner. The primary tool ATC uses is the flight clearance.
If air crews follow their clearances accurately and VFR traffic maitains adequate visual separation, there won't be any problems with separation being met.
TCAS can't issue a flight clearance. It doesn't know anything more than the traffic it can track via Mode C or Mode S interrogations. It's a tactical system that provides a backup when ATC or the crew makes a mistake and separation is lost. In this case, the TCAS is the automatic backup to resolve the loss of separation and avoid the collision.
The reason a TCAS RA must be followed, taking precedence over ATC, is that when there is a loss of separation immediate action must be taken.


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Can a pilot choose his co-pilot?
How are pilots paired in the duty roster?

Pilots choose their co-pilots from a pool (whitelist)? (Pilot has the final say)
Randomly, but taking pilot wishes into consideration? (Airline has the final say)
Ramdomly, without taking anyone's wishes into consideration?

In the case of (2) or (3), can a pilot refuse to fly with certain other pilots (blacklist), thereby having the power to overrule the airline?

At airlines with strong unions, what FOs will sometimes do if they are paired off with a Captain From Hell that they feel strongly about, is book off sick for that block.

Related question over on skeptics.se

As with everything it varies from airline to airline, but for the most part it is option 2).
Larger carriers will usually give pilots the option of blacklisting another if they wish, but smaller airlines with only a few dozen pilot combinations might just tell the pilot to suck it up.
The airline will also avoid placing a brand new FO with a brand new captain, aka ‘green on green’ pairings. And on top of this are things like fatigue duty limits, workplace agreements, seniority (bidding), currency and other considerations. Rostering is a very complex exercise.


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What are the differences between each ILS category?
I want to know the characteristics of the different categories of Instrument Landing System (ILS): CAT I, CAT II, CAT IIIa, CAT IIIb and CAT IIIc. What are the differences between them?
(This question only covers CAT III).

The category specs are easily found on Wikipedia. Here is a concise view of the ILS approach minimums, as far as the crew is concerned.

Cat I

DH: 200 ft (61 m) or more
RVR: 1,800 ft (550 m); at some airports 1,210 ft (370 m) is approved. For single crew operations, increased to 2,600 ft (790 m)
  orVISIBILITY 800 m (2,600 ft)

Cat II

DH: less than 200 ft and more than 100 ft (30 m)
RVR: 1,000 feet (300 m)

Cat IIIa

DH: less than 100 ft and more than 50 ft (15 m)
RVR: 600 feet (180 m)

Cat IIIb

DH: less than 50 ft (15 m) or none
RVR: 150 feet (46 m)

Cat IIIc

DH: No limitations
RVR: None

They are based on decision height and RVR (or visibility for cat I).
Note that there are no public transport aircraft certified for Cat IIIc as there is no use for landing without visibility and then staying on the runway because you cannot visually taxi to the gate.
The requirement for equipment certification accuracy itself can be found here.

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Why should lower limit of a Control Area (above 3000 ft) coincide with a VFR cruising level?
In ICAO Annex 11 it is recommended:

When the lower limit of a Control Area is above 900 m (3000 ft) MSL it should coincide with a VFR cruising level

Why should it coincide with a VFR cruising level?

Welcome to aviation.SE!

VFR cruising levels are always located between two IFR cruising levels. With half of the required radar separation minimum to each the IFR level above and below it.
For example:
required separation minimum: 1000ft
IFR lvl: 200
VFR lvl: 195
IFR lvl: 190
The sectors are split at VFR cruising levels to ensure, that both the upper and lower sector can use ALL of their IFR cruising levels while maintaining separation to aircraft that may fly at the highest/lowest level of the sector below/above.
Example:
Sector A is working from FL145-FL245
Sector B is working from FL245-FL355
Sector A may use every IFR cruising level within the sector (FL150, FL160, ..., FL240).
Sector B as well (FL250, FL260, ..., FL350).
As the difference between the highest and lowest level is always 1000ft, separation is ensured also between aircraft flying on top of each other in different sectors!

I agree this is the most obvious answer, but why would the ICAO not just define it as "the lower limit of a control area should be x500 ft". Why specifically mention VFR? I feel like there could be some less obvious reason behind it

@expeditedescent because in some cases it might be that the VFR levels are NOT x500ft (non-RVSM airspace). Before EU-RVSM was implemented all the division levels above FL290 had to be in implements of 1000, but still in between 2 IFR cruising level e.g.: FL310 and FL330 are highest and lowest usable levels in two sectors, they would then split at FL320

Again, I agree completely with your logic. I just think it's a bit weird that they are bringing flight rules into the mix, when the rule seems to have nothing at all to do with flight rules

@expeditedescent it for the sake of easiness. Check ICAO annex 2 Apendix 3, it‘ll be clearer then :)


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How can I calculate CLmax for a wing with multiple airfoils?
How should I do the math to find the CLmax of a wing that has more than one airfoil, each with a different CLmax?
I am trying to make a model with a portion of the wing tip with a airfoil that has a low Cm, but it has a low CLmax too. So...how can I calculate my CLmax?

The result depends on the wing twist and its taper ratio. You will never get the full 2D lift with a 3D planform, and how much you get depends on the details of your wing. The first step would be to apply Hans Multhopp's methods for calculating the lift distribution of wings (Subsonic Lifting Line Theory), and a search on Github shows a variety of codes to try. Factor in the different airfoils with local twist according to their zero-lift angle of attack.
Lifting line theory does not deal with flow separation and stall, so you need to interpret the results. Ideally, your airfoils will reach their stall angle of attack with a straight lift curve slope, and this stall angle of attack is reached at the same time over the whole span. This should maximise the lift from that wing. Adjust taper and twist until you reach this result (and subtract the twist from the differences in the airfoil's zero-lift angle to arrive at your geometric twist distribution).
However, if you also care about handling, leave some margin at the outer wing in order to have stall start first at the root. Also, you might want to add the effects of aeroelastic deformation in order to avoid unpleasant surprises.

I believe you just blend the two Cl values based on their proportionate share of the wing area.  Say 20% of the wing area is 1.4, and the other 80% is 1.6, so you use a blended value of 1.56 for a number to plug in when calculating stall speed.


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What are the dimensions of the A320's wingtip fences?
Does anyone know the size (relative to the wing) or have a schematic draw of these wingtip fences?

I wasn't sure what you meant by "A320-380" so I simplified the question to just the A320. If I got it wrong, please feel free to edit again to make it clear which model(s) you're interested in.

Did you find the dimensions of the wingtip fences as I am looking for them too!

Dimensional information can be found on Airbus's Airport and Maintenance Planning Documents for the entire product range.
You will have to do a little digging and calculation to find the sizes of the wingtip devices but they are there.
Specifically, on the A320, subject 2-3-0 lists the height of both of the bottom and top of the wingtip fences relative to the ground, and 2-2-0 provides dimensions from the top.

"Your answer is in another castle", or please provide a quote from the linked source that sufficiently answers the question.

Sharklet height is 2.43 meters according to the source above FIGURE-2-2-0-991-004-A01 and FIGURE-2-3-0-991-004-A01 lists the wing tip fence lower tip above ground as W2 = 3.78 and upper tip as W1 = 4.76 - so 0.98 meters....
Not very accurate through


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How does the position of the center of pressure change with the use of different high lift devices?
How is the position of the center of pressure affected by different high lift device deployments?
My guesses are:

When I use a high lift device that just increases the chord length, it moves backwards so that it stays at the same percentage of the length of the chord - correct?
When I use a high lift device that moves downwards, it has the same affect as increasing the AoA (or in fact: it does increase the AoA), so the center of pressure moves forward

Are those two guesses correct? Which other influences are there from high lift devices on the position of the center of pressure?

When I use a high lift device that just increases the chord length, it moves backwards so that it stays at the same percentage of the length of the chord

Only when the Fowler flap does not change the airfoil's camber, which is rarely the case. Normally, the flap will not only move backwards, but also rotate trailing edge down. A bit only for the take-off setting, but a lot for the landing position. This camber change will move the center of pressure back even if there were no surface increase, and the extension of the wing at the trailing edge will move the center of pressure further back again.

When I use a high lift device that moves downwards, it has the same affect as increasing the AoA, so the center of pressure moves forward

Again, no. The camber change dominates the position of the center of pressure, and it will move back since the wing now sports more rear loading (more lift is created at the rear end of the airfoil due to local camber increase).

This means that the movement of the centre of pressure rearwards cos' of downward deflection of aileron will induce more nose down pitching moment at the wing tip.CORRECT? @Peter Kämpf

@DavidTeahay: Ailerons move against each other, so the total change in the center of pressure is negligible. For the isolated section with a single aileron flap, you are correct, of course.


text:
Are there any training maneuvers that are only allowed when flying with a flight instructor?
My understanding of dual instruction training with a flight instructor and a student in a cockpit is, that certain maneuvers, that else would be not permitted, are okay, e.g. simulated engine failure and forced landing on some field (until doing a go-around from 100ft AGL). Is that correct?
For a trained and licensed pilot who currently isn't in training for a new license and just wants to train those things for him-/herself to stay trained, is there a way to do those maneuvers as well without a flight instructor in the cockpit? If that's not possible, does it change, if the single pilot is a flight instructor themselves? Or do they also need to have another flight instructor in the cockpit to train that?

I don't know about EASA, but in FAA-land there are no regulations that prohibit certain maneuvers to the supervision of a CFI, even engine-out simulations and forced landings. The only thing you need to abide by are regulations like acrobatic maneuvers and aircraft limitations... of course there is the catch-all "careless or reckless operation", but that is for the NTSB to decide.

far more likely the aircraft rental agency (usually the flight school) would restrict things they deem too likely to damage the aircraft if performed by people without extensive training (which should apply to CFIs).

Under FARS (so not EASA) an other exception is spins without a parachute.  See 91.307 (d)2.

Per the regs, no.  That’s the purpose of solo flying - to allow a student pilot to practice their skills on their own within a limited and safe environment.  However if you are renting the airplane, there may be prohibitions on certain maneuvers or operations eg spins, landing on unpaved runways, etc, so you will have to abide by those agreements.  Your CFI may place limitations on your solo endorsement at their discretion prohibiting certain flying for safety reasons which a student pilot must abide by.  As to any maneuvers required for a practical examination for a pilot certificate or rating, no there are no restrictions against flying them solo, provided they don’t conflict with other regulations eg operating an aircraft in controlled airspace on instruments without an instrument rating, etc.
Some schools also have limits as to what a student pilot can do solo.  I worked for a part 141 flight school once which forbade doing virtually every maneuver ie slow flight, stalls, steep turns, ground reference maneuvers, etc.  This just left the four fundamentals and changing heading, altitude and airspeed during flight.  They were training foreign students for the airlines and were pretty stringent.  I always though excessive.  But I’m sure they have plenty of bad memories where “you give ‘em an inch, they’ll take a mile”

What about e.g. the simulated forced landing vs. minimum safe altitude? Is it in the regulations that FIs are excempted from the minimum safe altitude for those maneuvers? Or is it okay for everyone to do that for training purposes?

Usually, unless you are near an airport, you will terminate a simulated forced landing no lower than 500 ft AGL in a rural area and 1000’ AGL in a populated area.  Again unless the maneuver conflicts with other regulations, there are no legal restrictions against it.

It's worth noting that being legal doesn't meant that something is safe or reasonable. FBOs and CFIs put restrictions on low-time pilots, beyond what the regs require, to keep them safe while they gain the experience needed to make their own risk assessments.

The only thing I can think of that requires someone else in the cockpit would be practicing simulated instrument flight in VFR conditions. In that case it doesn't need to be an instructor but just a qualified pilot.


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What does a 10nm/20nm circle on instrument approach charts mean?

A lot of instrument aproach charts (VOR or ILS) have a circle around the navaid. It can be 10/20 NM or in km, like the 20km circle in the image. What does this circle mean?

You may want to ask what it's used for rather than what it means, as I think you know it shows the distance that is 10 NM from the centre.

Yeap. Because, as I understood from the answer below (as Annex 4 says), it shows only radial scale. I thought, it has some additional function.

https://www.flightliteracy.com/instrument-approach-procedure-charts-part-two/
Normally, approach features within the plan view are shown to scale; however, only the data within the reference circle is always drawn to scale.

Wow, after 3 years!   Congrats.  Not sure that this is the fundamental reason for the reference circle, but-- definitely a good insight there,

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How do I get an FAA CPL from an EASA PPL?
Im looking into getting a PPL, but feel I need some more information since there are a lot of hours involved so I need to expand my idea of the different possibilities. I am from Sweden but work and plan to do the PPL in Italy, and as I understand it there will be no problem getting a PPL in Italy/Tuscany ( anyone heard of the Lucca Areo Flight School ? ) since I can get a PPL anywhere in Europe.
The plan is to move to the States in approx 2-2 1/2  years from now. There I will need to apply for an FAA PPL but will it be possible to continue on to the CPL in the US as a foreigner ?
As I understood from reading the threads I would need a PPL anyhow to do the CPL, and as I'm not 100% sure I'll be doing the CPL, is a step-by-step way preferable?

First, this question covers converting an EASA PPL to an FAA one. Based on what you said, I'd strongly suggest getting a full private license rather than a foreign-based one. That will probably require some flight training, which means you'll need TSA approval.
Next, when you have the private certificate then you can continue to CPL. In the US an instrument rating isn't required for a CPL but most people get it because otherwise the CPL is very restricted. You'll need TSA approval again for instrument training but not for commercial training. If you do multi-engine training at any point that's another TSA approval, by the way.
Assuming you have the time and money for all the training, your biggest issue will probably be legally entering and staying in the US. You need an appropriate visa or green card for flight training, plus the TSA approvals that I mentioned.

Thank you very much, so you would suggest I do the whole cpl including the ppl in US , instead of converting the EASA PPL to FAA one and then do the cpl in US ?
Sincerely Fred

In the other question I explained that if you already have an EASA PPL, you have two options to use it to get an FAA license:  validate your EASA license and get an FAA foreign-based license, or use your EASA experience and logged time to get a full, standard FAA license. I think the second option is better.


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What is a TAPS combustor and how does it differ from any other annular combustor?
I found out that the GEnx engine uses a TAPS (Twin Annular Premixing Swirler) combustor. Is it a single or dual annular combustor ?

Probably a duplicate of your other question: What is the difference between single and dual annular combustor?. As you don't have yet selected an answer, you could just request about TAPS particularity in the same post.

See this question.  It is a single annular combustor (one ring of full nozzles).  I think that "twin" refers to the fact that within a single fuel nozzle, there are two different fuel streams: the main stream and the pilot stream.  This video might also help explain it a bit.


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How does the nitrogen generation system in a Boeing 777 work?
How does the nitrogen generation system in a B777 work and how does it differ from that used in a B787? Any diagrams will be appreciated.

The 777 system
As RAC points out, "nitrogen-generation system" is somewhat of a misnomer. All the system does is remove oxygen from the ambient air and blow the rest into the center fuel tank; there is no chemically "new" nitrogen created. The 777 architecture is basically the same as the schematic that mike provides; in fact, all bleed-air aircraft use pretty much the same system. For example, here is an Airbus schematic:

Parker has a breakdown of the major components. Summarizing it and filling in some gaps, we have:

Ozone (O3) converter: Basically a catalytic converter that converts triatomic oxygen (ozone) to biatomic ("regular") oxygen to protect other elements in the system from oxidation.
Heat exchanger: Cools the bleed air (which is really hot when it comes off the engine), again to protect other elements of the system and increase their effectiveness.
Air separation module: Physically removes almost everything but the dinitrogen (N2) in the air. This separation is accomplished by running the air through semipermeable fibrous tubes; because almost all the non-N2 molecules present are smaller than the N2 molecules, those smaller molecules pass through the membranes and are exhausted overboard, leaving only the nitrogen to exit into the fuel tank.
Other: There are various filters to protect components from particulate matter, valves to control the timing and rate of flows, controllers to control the valves, and sensors to inform the controllers.

The 787 system
The 787 is a no-bleed airplane, which means that instead there is an electric-driven compressor to supply the NGS rather than bleed air. This setup requires a slightly different heat-exchanging architecture, but the overall function and operation of the primary system components are similar.

On the 787 it's NOT a nitrogen generation system, it's an oxygen depletion system. If the air starts at 77% N2 and 21% O2, and you take half the oxygen out, you end up with 87% N2 and 11% O2. The whole idea is to reduce the balance of fuel fumes v oxygen above the fuel, so that it won't burn.
On the 787, air from the Cargo Heat system is passed through a catalytic convertor (like your car), which takes out some of the oxygen. The reduced-oxygen/enriched-nitrogen is then fed to the fuel tanks like a vent system.

The NGS uses proprietary tech to separate nitrogen enriched air from bleed air. Diagram attached.

...and why are Boeing allowed to keep technology that would greatly increase aviation safety secret?


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Wing pitching moment decomposition into two terms
Summary: Assuming constant angle of attack $\alpha$, speed $V$, and lift $L$ applied at the center of pressure CP (whose position is $x_{CP}$), the distance $(x_{CP} - x_O)$ between the moment reference point $O$ (whose position is $x_O$) and $CP$ determines the sign (positive, negative or zero) of the pitching moment as long as $L\neq 0$:
$$M_O = (x_{CP} - x_O)L$$
However, the moment $M_O$ depends on $\alpha$, i.e. $M_O=M_O(\alpha)$, since changing $\alpha$ while keeping the reference point $O$ position constant, varies the magnitude $M_O$ since both lift $L$ and the position $x_{CP}$ vary.
Interestingly, when $O=AC$ and $x_O=x_{AC}$, the moment does not change with changing $\alpha$:
$$ M_{O}(\alpha) = M_{AC} = (x_{CP} - x_{AC})L = constant$$
We can move lift $L$ from its application point $x_{CP}$ to point $x_{AC}$ while adding a constant free pitching $M_{AC}$ whose magnitude is $(x_{CP}-x_{AC})L$. The moment $M_{AC}=0$ for symmetric wings and $M_{AC}\neq0$ for cambered wings.
In the case of a cambered wing, the equation
$$ M_{AC} = (x_{CP} - x_{AC})L$$ predicts that $$M_{AC}=0$$ when lift $L=0$. However, we know that a cambered wing has a a constant nonzero moment $M_{AC}$ for any $\alpha$ even when $L=0$ at the zero-lift $\alpha_0$. How do we transform the equation for $M_{AC}$  so it becomes equal to the sum of two terms, one solely due to camber and one solely due to lift:
$$ M_{O}(\alpha) = M_{AC} = (x_{CP} - x_{AC})L= M_{camber}+M_{lift}$$
where the term $M_{camber}\neq 0$ for any $\alpha$?

The center of pressure moves to infinity as lift approaches zero for a cambered airfoil. No need to reduce the moment to zero just because lift disappears.

Interestingly, when $O=AC$ and $x_O=x_{AC}$, the moment does not change with changing $α$

Yes, but only if your lift is also applied at the AC! That's the whole point of the aerodynamic center. You replace the pressure distribution with a lift force (actually a resultant force, but let's neglect drag here) and a moment. If you place the lift force at the center of pressure, then the resultant moment about that point is zero. Then, the moment about any other point $O$ is only "created" by the resultant force. So in that case indeed:
$$M_O = (x_{CP} - x_O)L$$
If you now place the lift at your AC and also O=AC, then:
$$M_O = M_{AC}$$
This $M_{AC}$ is independent of $\alpha$. If you now move to any different O, you get:
$$M_O = M_{AC} + L (x_O - x_{AC})$$
The first term $M_{AC}$ is what you are calling "$M_{camber}$" and the second term is your "$M_{lift}$".

I understand your explanation. On my book (I can attach page) the  moment $M_{AC}$ due to lift $L$ about the aerodynamic center $AC$ is constant even when $L$ is applied at the $CP$ and even without the vector trick of transferring lift $L$ so it acts at $AC$ with an added constant couple moment. If $L$ acts at $AC$, the lever arm is zero but the free couple moment is nonzero.When $\alpha$ increases, $L$ increases and the lever arm $(x_{CP} - X_{AC})$ decreases.So I agree that $$M_O=M_{AC}$$ But, I think the decomposition into the 2 mentioned moment terms is only possible if $L$ acts at $AC$.

In the case of a cambered wing, the equation
  $M_{AC} = (x_{CP} - x_{AC})\cdot L$ predicts that $M_{AC}=0$ when lift $L=0$.

No, it doesn't.
It only predicts that either $(x_{CP} - x_{AC})$ approaches infinity or that lift decreases to zero. Both conditions will satisfy $M_{AC} = (x_{CP} - x_{AC})\cdot L$, but only the first one will satisfy $M_{AC} = const.$ as well.

Plot from XFLR5 (own work). The overlaid line shows the center of pressure on a wing with washout at small angle of attack where the inner wing creates positive lift while the wingtips create negative lift. At approx. 75% of span the sign of the local lift force changes (= local lift is zero) and the center of pressure switches from negative infinity to positive infinity (apologies for the line not really going to infinity because it has been calculated only at discrete points along the span, but I hope the plot gets the point across).
Why would this happen? At $L = 0$ there is both positive and negative lift along the wing chord. An airfoil with positive camber will show negative lift in the forward part of the airfoil and positive lift in the rear part. Even if their sum is zero, there is enough local lift to create a sizeable pitching moment. Below you see the XFOIL results for a NACA 4409 airfoil. Note that lift is effectively zero while the pitching moment remains unchanged.

Blue is upper side pressure while red is lower side pressure relative to static pressure. The suction peak near the nose has traveled to the bottom side due to the low angle of attack while the rear part of the airfoil, which is less affected by AoA changes, still shows positive lift from camber. For illustration, the same thing with arrows indicating local pressure:

So as the net lift $L\rightarrow 0$ the location of $x_{CP}\rightarrow \infty$: the force decreases but the lever arm increases keeping $M_{AC}$ constant. The nose down moment is caused by a force couple: the lift closer to the LE  is directed down and the other lift force is directed up. I didn't know the positive lift was at the wing inner region while negative lift at the outer area. The force couple would also be predicted for an infinite span wing, right? What causes the lift in the front to b downward? The higher pressure at the top surface relative to the wing's lower surface. But why?

@BrettCooper: Remember that zero-lift angle of attack is negative: The negative lift near the leading edge comes from the negative angle of attack while the positive lift near the trailing edge is from camber. The changing lift of the sample wing in the plot comes from the change in incidence over span (washout).

Thank you, Daniel and Peter.
Just to wrap things up on this topic, the two-term moment decomposition for $M_O$ is mathematically possible when:
a) $L$ is moved to act at $AC$ instead of $CP$.
b) the moment $M_0$ generated by $L$ acting at $AC$ is calculated about a moment reference point $O\neq AC$
If a) and b) are satisfied, the pitching moment $M_O$ becomes:$$M_O= M_AC+L(x_O-x_{AC})$$ which is the sum of the two moment terms $M_AC$ and $L(x_O-x_{AC})$
However, if $L$ is not moved to $AC$, the moment expression calculated for $L$ acting at $CP$ about an arbitrary point $O$ is given by $$M_O= (x_{CP} - x_{O}) L$$
and this last expression cannot be converted into a sum of two moment terms (one solely due to camber and one solely due to lift).

The moment reference point is the quarter chord point / neutral point / aerodynamic center (AC), so if lift is applied at AC, it creates no moment. To faithfully reproduce reality (where lift would be summed up at some variable location along the chord), you need to add a moment (offset moment - you should know this link!). When lift is shifted to the AC, this moment is constant over angle of attack. Please let me know what is unclear in the linked answer!

Thanks. I will study that past thread and post. When you say that there is no moment, you are referring to the lift generated moment when the moment is calculated about $AC$. When $L$ is applied at $AC$, the moment about an arbitrary point is $$M_{O}=M_{AC} +(x_{O}-x_{AC})L$$ If $O=AC$, $x_O=x_{AC}$ and $$M_{AC}=M_{AC} +0$$ since the lever arm $(x_{AC}-x_{AC})=0$

What you say is correct, but your symbology is confusing. Why don't you call the constant moment from camber $M_C$ and avoid calling two different things $M_{AC}$ ? Then you can write $$M_0 = M_C + (x_0 - x_{AC})\cdot L$$ and $$M_{AC} = M_C+0$$ This would probably look less confusing.

Great. Thanks. I would think that, experimentally, in a wind tunnel setup, the best way to empirically measure $M_C$ would be to support the wing from the point $AC$, i.e. use $AC$ as the fixed pivotal point about which to support the wing. Once we find the AoA where $L=0$, the moment experienced by the wing is only $M_{AC}$...Still, not sure how to measure $M_{AC}$ empirically anyway...


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If an aircraft flies straight ahead, does it follow an orthodrome/great circle?
I understand that an orthodrome (line on a great circle) is the line with the shortest distance between two points on a sphere.
If an aircraft flies straight ahead, i.e. there is no wind, the rudder is not actuated and the aircraft is parallel w.r.t. the surface of the earth underneath it, will it follow an orthodrome/great circle?
I also understand that a loxodrome is the line that one follows when keeping a constant heading. This is the point where I'm confused, because wouldn't that mean that when flying straight ahead/along an orthodrome, the heading/bearing would constantly change? In other words, does this mean that one needs to turn in order to keep a constant heading?
EDIT: My question is not "which line does one follow when keeping a constant heading" (a loxodrome, I know that) but rather "what line does one follow when flying straight ahead (in the hypothetical case of no wind and no control inputs)?"

Welcome to Aviation.SE, this is a good first question! I would recommend taking a tour of the site and reading the [help] as well to help you understand the site a bit better.

Closely related, maybe a dupe? Also this question.

@Pondlife That question does say again that flying with a constant heading makes you follow a loxodrome. But my question is related to flying an orthodrome and what that means for the heading displayed inside the aircraft.

Possible duplicate of What trajectory do I fly if bearing angle is constant?

What effect does the Earth’s rotation (I.e. Coriolis force) have?

@JimGarrison, none because the discussion is based upon ground track and does not address winds in any way. The problem is actually a geometric problem but if we assume the Earth to be spheriod it maps to navigation on the surface of the sphere.

I don't think that's 100% correct. If I start at the equator and fly due north I have over 1000 mph eastbound velocity as well, which will have an effect on my ground track if I maintain the heading as I go north.

To add to your confusion :-( in general "great circles" on real Earth are not closed, they are spiraling geodesics.

@mins But isn't a great circle by definition closed? I do understand, that geodesics do not form closed circles on an ellipsoid. But when cutting an ellipsoid with a plane, doesn't this create the great circle path, which is closed? Or does it not even make sense to talk about great circles on an ellipsoid?

@JimGarrison But what does this eastbound velocity cause? The atmosphere moves with you and when assuming no winds at all (I know, we are neglecting the Coriolis effect), this shouldn't matter (your reference frame rotates with you).

@Daniel the second thing you said: it is not meaningful to speak of great circles on ellipsoids. When you cut an ellipsoid with a plane going through the center, what you obtain looks like a great circle, but it is not.

In complement of @Federico 's comment: It means that if you go from A to B to A flying in the same direction, with A and B at different latitudes, then the curve isn't a circle, and the points composing this curve aren't even coplanar (to be fair: the dispersion is not very large either). Great circles are geodesics (shortest paths) on a sphere only, not on real Earth.

Probably useful for digging further: Comparison of great circle sailing, great elliptic sailing, and geodesic sailing

If you start flying as you say, and unless the trajectory is along the equator, the Coriolis forces will soon deviate the initial trajectory, that will depart from the orthodrome...

Re "If an aircraft flies straight ahead, i.e. there is no wind, the rudder is not actuated and the aircraft is parallel w.r.t. the surface of the earth underneath it, will it follow an orthodrome/great circle?"  -- I don't understand the motivation for the question.  It seems completely obvious that this would be true.  Unless you want to get into deviations due to the Coriolis effect -- only significant when flying very fast over very long distances.

Anyway, certainly there's no reason to assume that the heading would tend to stay constant even if that forced the aircraft to deviate from the great circle route.

When flying an orthodrome, with few exceptions, the heading will be constantly changing. In theory the change is continuous, but in practice most aircraft are quantatized to the nearest degree.
An exception are missiles, where because of a higher speed, having a more precise heading is more critical.  Therefore most longer range missiles will internally use a higher granularity in the heading (like 0.01 degree or less).
To be clear, a loxodrome is a course which crosses all meridians of longitude at the same angle, and has a constant bearing measured to true or magnetic north.  Loxodrome are also called rhumb lines. All loxodromes spiral from one pole to the other pole, except longitudinal loxodromes.
An orthodrome is also called a great circle route, and is characterized with heading changes (for most headings) to allow the vessel/aircraft to fly the shortest path along the surface of the earth to get to another point on the earth. Assuming that the earth is a sphere, an orthodrome is defined by a plane which goes through the center of the sphere, and the curved lines formed by the outer portion of the sphere intersecting the plane form what is known as a great circle route.  The equator and meridians of longitude and their inverse lines on the other side of the sphere, form orthodromes. In those examples, travel on the equator is a constant heading of 090 or 270. On longitude lines, the heading is either N or S, until polar passage. An equatorial orthodrome cannot be a loxodrome. A longitudinal orthodrome is a loxodrome, although a rather uninteresting one.
It all seems rather straight forward now, right? There are wikis on both rhumb lines / loxodromes and also great circle routes or orthodromes. The graphics in them may help understand things.
https://upload.wikimedia.org/wikipedia/commons/thumb/0/04/Great_circle_hemispheres.png/220px-Great_circle_hemispheres.png
https://upload.wikimedia.org/wikipedia/commons/thumb/d/d6/Loxodrome.png/220px-Loxodrome.png
Addendum to readdress question from OP:

If an aircraft flies straight ahead, i.e. there is no wind, the rudder
  is not actuated and the aircraft is parallel w.r.t. the surface of the
  earth underneath it, will it follow an orthodrome/great circle?

The answer is always yes.
What the OP appears to be asking is whether the airplane, flying with only an inertial reference (NOT an inertial guidance system) and a fixed distance above the surface of the earth, can be accomplished without a change in direction, except for the circling of the earth.
So to explain this, let's call the earth a sphere for this discussion. If a great circle route is extended, it will scribe a line which wraps around the sphere, and divides the sphere into two identical half-spheres.  Those half-spheres can be made with a single directional cut of the sphere. If you will, the sphere is split in half by a plane (geometric type, not aeronautical type) and the circle formed by that plane is the great circle route wrapping around the sphere.
The heading of an airplane flying a great circle route will with few exceptions, be constantly changing. The exceptions are when the plane is the equator or is a meridian of longitude and the corresponding reciprocal meridian.
So once again, when the path of the aircraft is constrained to the surface or some fixed distance above the surface, the inertial direction relative to the surface of the sphere, will remain a constant direction. Since only great circles which pass through the poles or travel the equator have one axis fixed relative to lat/long of the earth, they will have constant headings. The ones which are polar will have heading flips at the poles. All other orthodromes will have continuously changing headings.
There is just one more aspect of navigation that I would be remiss to not mention, and that is Transport Wander, which may be observed on a heading indicator on an aircraft, and is the function of the sin(track angle) * delta longitude/flight hours * tan(latitude)/60. The polarity changes with east vs west and northern vs southern hemispheres.
To the OP, I am sorry that I misinterpreted your question, and for the resultant confusion.

The line of constant heading isn't a straight line by any definition.

"When flying an orthodrome, with few exceptions, the heading will be constantly changing."
Alright, that makes sense. But when flying without control inputs and without wind, will the aircraft follow this orthodrome?

@mongo What do you mean with "it will not fly a path on it's own"? It will have to follow some path right? Like a straight line along the ellipse (geodesic), if I got it right.

Alright, thanks to everyone who answered, now it appears more clear! With the combination of the answer I seem to finally understand it.

There isn't a perfect answer to this, but effectively yes.
Anything that moved in a completely straight line would of course leave the curved  surface of the Earth and go off in to space. And realistically no vehicle of any kind could perform the (lack of) manouvers described above with no control changes.
To perform the best approximation of 'straight flight' while staying on the surface of the Earth would mean deviating from the true straight line only in a direction perpendicular to the Earth's surface (i.e. "up and down"). This in turn means staying in the plane defined by containing the vector of direction of initial motion and also the centre of the Earth. The intersection of the Earth's surface with this plane - i.e. the path the craft would actually follow- would be the great circle. And in answer to the last part, yes, when flying a great circle the heading constantly changes.
EDIT: This does, of course, assume a reference frame relative to the Earth, as pointed out in comments..

Upvoted, however, since your reference frame rotates, the great circle would be disturbed by the coriolis force (unless you are moving in the plane of rotation, i.e. heading E/W along the equator).

Your explanation with the "plane defined by containing the vector of direction [...] and also the centre of the Earth" actually helped me quite a lot!
So just to recap: without control inputs or wind, one will stay on the great circle, with a constantly changing heading?

@Daniel assuming no wind and a trimmed aircraft then yes.

If you go straight north-south, you follow longitude lines which are straight.
If you go east-west at the equator, that latitude line is straight.  All others hit the earth's surface at a bit of an angle, and so they are curved.
Imagine you follow the 89 deg 59 minute parallel, which is 1 minute (1/60 degree) south of the North Pole. Like every parallel, it's a circle.    It's close enough that at altitude, you could see the north pole.  You could also see past the north pole to the 89deg59' parallel ... on the other side of the North Pole.  The parallel you are on! You'd be over there in a few minutes, heading 090 the whole time.  And obviously you'd have to turn to get there, since you are flying a circle.
At lower parallels, the effect isn't nearly as extreme, but it's still a turn.
If you fly diagonally, at a heading not a multiple of 90, then east-west travel is part of your path, so a slight turn is part of the heading.
How can the path be curved if you are flying straight?  It's not... The location grid system is curved because Earth is curved.

Alright, that's a nice explanation for why maintaining a constant heading requires turning!
So I guess from this one can deduce that flying completely straight means a constant change in heading?

@daniel precisely.


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What is the transient bleed valve in a gas turbine engine?
What is a transient bleed valve and what's its purpose ? Also, can you explain the purpose of bleed valves in general?

Which particular engine or aircraft are you asking this question about?

@KorvinStarmast Ge 90

The high pressure compressor in a typical engine has many stages (e.g. GE90-115 has 9) all running on the same shaft, and thus all at the same speed. The problem is that sometimes the flow/pressure that the front stages are producing does not match the flow/pressure that the back stages are able to accept. When this happens, the compressor will stall (this is bad).
This can happen in several different situations, and one of them is a quick transient.  For example a very quick acceleration from idle to full power.
During this quick transient, the front stages might be trying to pump too much air versus what the back stages are able to accept. Eventually, everything will even out, but for a few seconds there may be a mismatch.
There are several different ways that engine designers can accommodate this situation.  One is a transient bleed valve.
When the engine performs a very quick acceleration, the transient bleed valve is opened and some of the air from the compressor is just dumped overboard.  When the engine reaches a steady operating condition, the valve is closed.

Dumped over or into the turbine. From this great presentation (slide 67 and next ones) about CFM56-5: "The Transient Bleed Valve (TBV) improves the compressor stall margin during transient and start conditions. The Transient Bleed Valve unloads the HP compressor by discharging stage 9 HP compressor air in the LP turbine cavity." (see this image)

can you explain the purpose of bleed valves in general?

See How do variable bleed valves prevent surge? why bleeds are used to prevent the compressor to stall. An answer to that question already hints to the use of bleed-off in transient operation. This answer will describe the transient performance in more detail by using the compressor characteristic/map which describes the relation between pressure ratio (PR, on the vertical axis), corrected mass flow (Wc on the horizontal axis), the efficiency (η, contour lines in the graph) and constant corrected and referred speed lines (Nc in the graph) using the Beta lines (What are the "beta lines" of a gas turbine engine component?) for numerical stability when used in gas turbine simulation performance software.

What is a transient bleed valve and what's its purpose ?

This answer by DanielK already explains in words what happens, this answer builds upon that answer to give a little more insight in the problems gas turbine developers encounter to design stable engines for an certain expected life usage.
When you aren't taking the engine limits into account and bluntly decrease or increase fuel too abruptly, the engine can get into regions in which is cannot operate or consumes too much life or even destroys the turbine. E.g. a SLAM deceleration/acceleration operating line (blue solid line) for a small turboprop engine may look like the following in the compressor characteristic:

The image clearly shows that the acceleration from lower left to upper right crosses the red surge line on the left. Do note that in simulation software, the digital representation of the compressor characteristic is extrapolated beyond the surge line, in real life, the compressor is unable to operate on the left of this surge line. The operating point of the gas turbine should even be a few percentages to the right for stability.
If we do not solely look at the surge margin, we see that the combustor exit temperature (TT4) will be too high also:

To prevent the operating line to cross the surge line to the left during transient operation, several techniques are available:

use of Compressor Variable Stator Vanes (VSV) which effectively moves the surge line to the left in the compressor map,
use of bleed-off to move the operating line to the right of the surge line increasing the surge margin,
scheduling fuel.

Optimizing the fuel addition (gradually addition, e.g. fuel flow schedules based on the burner pressure) to the cycle and the use of VSV's and bleeding compressor air will cause the engine to stay within limits, see the dashed operating line:

In addition to transient bleed valves, bleed valves in general are also used to duct high pressure air for different uses, either for the engine or the aircraft. Bleed valve air can be directed to the leading edge of the engine nacelle and wings for de-icing or anti-icing.
Bleed valve air can also be used to cool different parts of the engine (combustors, turbines, etc.), and for pressurizing the bearing compartments (?). My understanding of jet engine bleed air is from older engines, so current engines may be different. I'm fairly certain it's still used for de/anti-icing.
Bleed air has also been used to start engines on multi-engine aircraft. Start one engine on each side, then duct bleed air to the starter of the other engine.
On some modern engines the use of transient bleed air has been supplanted by adjustable compressor guide vanes and other methods of managing compressor operation.

You are correct.  Bleed air is still used for anti-icing on virtually all current engines (not sure about the 787 since that is supposedly "all electric", but everything else still uses bleed air for anti-ice)


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What exactly is considered "instrument time" for an instrument rating?
I have read in several sources that you are supposed to log instrument time when you cannot orientate the airplane by outside, visual cues alone. This seems clear enough. But I've also read that roughly 10% of flight time should be on instruments. Now, the instrument rating requires 40 hrs on instruments or in poor visibility conditions, so can we infer that pilots would receive their rating after approximately 40x400 = 16,000 hrs? Is this estimate approximately correct? Also, does all of this time require an instructor?

In what country? I think it's safe to say that pilots can get instrument ratings in less than 16,000 hours (40*400)... I know a few who got an instrument rating in less than 125...

You can use simulated instrument time to qualify for an instrument rating, so you can do it much more quickly than you estimated. But for details of what's allowed, when you need an instructor etc., we need to know which country or regulations you're asking about.

I think you are conflating two different things. Most IFR rated pilots have very little actual instrument time. Even airline pilots who fly in all kinds of weather have a low percentage of their time in the clouds. A lot of it has to do with where they fly—at over 30,000' they are often above the clouds. I would imagine that it varies a lot with the type of flying that the commercial pilot does. A freight hauler in the Northeast probably has a lot more instrument time than one in Arizona. Likewise, someone who flies their small aircraft a lot on business might have relatively more actual instrument time simply because they will be flying in the weather rather than above it.
If you look at the instrument requirements in CFR Part 61, the training requirements for the Instrument say "…hours of instrument training using a view-limiting device…". So it is possible to get your instrument rating with 40 hours of flying "under the hood" and never fly in actual instrument conditions. (You can also get some hours in a flight simulator.)  And when you take your practical test for these ratings, you will be flying under the hood and not in actual instrument conditions.
The same language is used in the Commercial rating as well. In fact, you could have a CFI who has never flown in actual instrument conditions teach you how to fly IFR!
To get an instrument rating you need 40 hours of hood time. Only 15 hours of this is required to be with an instructor. Lots of people save money on their rating by flying with a pilot friend or fellow student.
Once you have your instrument rating, you need to maintain currency by flying 6 approaches, holding patterns, and tracking courses in actual instrument conditions or with a view-limiting device. If you listen to the radio on a nice weekend, you will hear lots of pilots requesting practice approaches to maintain currency. Usually they are flying with a friend rather than an instructor.

Instrument time is, as you say, when flight is conducted solely by reference to instruments. This can further be broken down into actual and simulated time. Actual time is when you are flying in cloud or visibility below VFR minimums and cannot navigate by visual references. Simulated time is when you artificially limit your outside view and requires a safety pilot.
The 10% rule is a rule of thumb of what instrument time you may see in the normal course of flying. It is not applicable to training. During training you may fly 95% of your time as simulated instrument (everything but taxi, takeoff and landing). For this time you need only another rated pilot who can be a safety pilot. It does not need to be an instructor.
If you have a private pilot rating and nothing more you can get an instrument rating in tens of hours, not hundreds or thousands of hours.


text:
Why can't I find a tail number in the FAA registry?
I'm reviewing my pilot logbook in preparation to move everything over to my EFB logbook. As part of that process, I have been looking up the various aircraft I've flown in the FAA registry to get details about those aircraft to make my log complete (my digital pilot logbook tracks more information on an aircraft than my paper logbook and I'd like to have the details for my own nostalgic purposes).
One particular aircraft I flew in 2005, N7635A, is nowhere to be found. Clearly the plane existed - I flew it! And it's in my logbook 3 times complete with my CFI instructor's signatures in those entries, so he seems to have thought the plane existed as well.
When I look up N7635A, I get a Cessna 180 (circa 1960s). I never flew a 180 (seems like I would have noticed that since the rest of the fleet was made up of 2002 - 2004 Cessna 172SPs and they all had that new plane smell). https://registry.faa.gov/aircraftinquiry/NNum_Results.aspx?NNumbertxt=7635A
My first thought was maybe the FAA drops registration records if a plane leaves the country - but that Cessna 180 is deregistered because it was exported to Canada. Also, another plane I flew, N65460, is also deregistered but still has a record with the FAA: https://registry.faa.gov/aircraftinquiry/NNum_Results.aspx?NNumbertxt=65460
I have tried different combinations:

N7635A - 1960 Cessna 180 (wrong plane)
N763SA - 1979 Cessna 152 (wrong plane)
N7G35A - not valid
N7G3SA - not valid

N60432 is another mystery 172S in my logbook but that's from 2013 and I only have a single entry for that one.
Do I have a correct understanding of the FAA N-Number registry? Am I wrong to think it's a complete record of all FAA N-Number registrations (going back to at least 1960)?

My guess is that the owner of the plane might have liked the tail number and moved it to his new plane. The FAA can also recycle old tail numbers for planes that are no longer flying. But since this is for an older plane, that isn’t likely. A famous example of this is the John and Martha King run-in with gun-toting law enforcement. https://www.avweb.com/avwebflash/news/John_Martha_King_Held_At_Gunpoint_203205-1.html

I would have expected the N-Number registration to show the prior aircraft associated with that tail number. For example: https://registry.faa.gov/aircraftinquiry/NNum_Results.aspx?NNumbertxt=401

Not that I am aware of. I am curious about unusual aircraft that I see on the field and frequently look up the tail number to find out what kind of airplane it is. I don’t recall ever seeing one with a previous aircraft listed.

@JScarry - interesting that in that article, the Kings say they "don't look like airplane thieves". I wonder what airplane thieves do look like... :)

@FreeMan Some of them look like this. https://www.cbsnews.com/news/colton-harris-moore-the-barefoot-bandit-is-baaack/

@JScarry huh... he doesn't look much like a plane thief to me, but what do I know...

Perhaps the FAA is just inconsistent about keeping histories for multiple aircraft with the same tail number. For example, N401 has a history of aircraft for that tail-number, but that doesn't mean they all do (?)(https://registry.faa.gov/aircraftinquiry/NNum_Results.aspx?NNumbertxt=401). And perhaps they're just inconsistent with keeping records of deregistered tail-numbers (?). For example: https://registry.faa.gov/aircraftinquiry/NNum_Results.aspx?NNumbertxt=65460

I talked on the phone with the FAA concerning registration of my aircraft 2x in 2021. It took a while to get someone, but it was worth it. Skip the middle man and call the FAA. I don't like to see answers like, it could be, maybe this/maybe that. That should be a warning to your common sense. You will never feel relaxed until the FAA is contacted.

@jpoveda: They seem to be pretty inconsistent even with the most recent (or, sometimes, only) aircraft to hold a particular tail number: N999UA is missing its certificate-issue date, while N600XL is completely missing the data for its most recent aircraft.

I think wrong to believe N-numbers are not recycled.  The process has changed, but now we (owners) must register every two years (I think two years).  //see comment below, registration update is every 3 years and $5 which is background noise compared to an annual

I would imagine they'd be recycled, but wouldn't the FAA N-Number record have a reference to prior registrations? I've seen that on some of the registrations. For example: https://registry.faa.gov/aircraftinquiry/NNum_Results.aspx?NNumbertxt=401

I have no glib answer for you.  The FAA works in mysterious ways.  I get a snail mail notice (I think every other year) asking if I still own an airplane and demanding a small payment.

@guycole It is every three years and currently can be done on-line for $5.

This doesn't actually answer my question, though I'm thankful for your insight. My question is why can't I find a specific tail number and whether or not the FAA keeps historical records of all aircraft that have used a tail number. Your answe partially answers the first part of my question only if the assumption is that they do not keep historical records. It appears they do keep historical records in some cases: https://registry.faa.gov/aircraftinquiry/NNum_Results.aspx?NNumbertxt=65460


text:
What ground-based, airline job in the 1940s could have involved radio communications?
My mom worked for United Airlines in the mid 1940s in some communications capacity: she communicated by radio with United aircraft only, and she wasn't an air traffic controller.  She went to school in Kansas City in 1942 for communications.
Could anyone help me figure out what her job would have been?

Is it possible she studied Communications such as public relations, advertising, etc or do you know that she worked with some sort of radio system?

she did work with a radio system. she said she communicated with the United planes only.

Did she work at an airport or in an office building somewhere else?

Trinidad, CO was a Continental Airlines DC3 stop long ago. The airport has a huge complement of legacy radio gear related to airline operations. My understanding at the time was that it was a weather station operated by the airline. The terminal building still had the ticket counter, scale, airline offices along with a 1950s soda fountain last time I visited. It was like a time warp.  http://blogs.denverpost.com/thebalancesheet/2015/08/28/csbr-road-trip-warning-trinidad-airport-vintage-airport-stuff-may-cause-excitement/13566/

Perhaps she was a flight dispatcher?

A flight dispatcher (also known as an aircraft dispatcher, airline
dispatcher, flight follower or flight operations officer) assists in
planning flight paths, taking into account aircraft performance and
loading, enroute winds, thunderstorm and turbulence forecasts,
airspace restrictions, and airport conditions. Dispatchers also
provide a flight following service and advise pilots if conditions
change. They usually work in the operations center of the airline.

-Wikipedia

In addition to the duties that @Steve V mentioned, the dispatcher was the person who would provide ground support, such as ordering fuel, and filing  flight plans (radio from the plane, phone to FSS).  They were also liaisons between maintenance when there was a problem, and would work out equipment changes, etc.

This would have been better suited as a comment on Steve V's answer...


text:
Has anyone attempted to board a flying aircraft mid-air?
A few movies (e.g. Air Force One, Executive Decision) feature a story where people board a flying aircraft from another flying aircraft.
Has anyone ever attempted, either via docking two aircraft together, using cables, parachuting on top etc., to get from one flying aircraft to another mid-air?

Sure, people have tried all kinds of crazy stuff. For example, here's a story about how two pilots switched planes in mid-air.

That sounds like a good answer.

When you say "aircraft" do you also include blimps? They often had parasite aircraft that could dock/undock and exchange pilots.

Related: Parasite Aircraft (Wikipedia)

This video shows a stunt in which base jumpers jump from the ground into a plane.

I remember performers walks from one hot air balloon to another via a wood board bridge. So that fits the definition of both "boarding" and "aircraft".

Sure. It was regularly done by the aerialists or barnstormers of flying circuses during their heyday in 1920s.

Barnstormers performed a wide range of stunts. Although many of them handled all their own tricks, others became specialists, either stunt pilots or aerialists. Stunt pilots performed daring spins and dives with their planes, including the well-known loop-the-loop and barrel roll maneuvers. Aerialists, on the other hand, performed such feats as wing walking, soaring through the air with winged costumes, stunt parachuting, and midair plane transfers.

These were mostly done using army surplus biplanes after WWI. The performers boarded aircraft from a variety of vehicles, from cars to speedboats. This ticket for one of the performances clearly mention the midair plane transfers.

Ticket for a flying circus; Image from all-things-aviation.com
As time went by, lack of cheap aircraft and tightening federal regulations put an end to barnstorming. I'm not aware of anything like that in modern times. Anyway, mid-air transfers are not easy and are quite dangerous, as Simon Crane found out while shooting for Cliffhnager.


text:
Why do some 2 seat fighters have side by side seating, while others have the pilots inline with each other?
In some multi seat fighters, like the F-111 Aardvark the pilots sit next to each other, like so:

While in other multi seat fighters (most of them it seems) the pilots sit with one in front of the other, like this F-15E:

Why do some fighters have the pilots in a line like that?  Is there an aerodynamic or performance advantage that comes with having them in a line?  Or maybe it's so they can have a bubble canopy, which perhaps affords a better view? It seems it would be easier to work together when they are side by side.
So why do some multi seat fighters have side by side seating, and others are in a line?

My naive guess is that a narrower fuselage is aerodynamically more efficient in general. i.e. A large L/D ratio. So I'd go with front & back seating as my default guess for a good design. e.g. The same reason why kayaks mostly have the people in one line & not side by side.  Probably doesn't hurt to give a smaller projected cross section to enemy fire though with modern weapons I doubt this matters much.

I don't think there are too many side by side seating fighters, honestly.  The front/back setup is probably more aerodynamic, whereas the other provides equal visibility for both pilot/CSO.  EDIT: on second thought, the pilot probably also has better visibility with no one next to him.

F-111 is numbered like a fighter, but calling it a fighter is a long shot. It is an attack aircraft/light bomber. Which F-15E is as well, but F-15E is based on F-15, which was designed as interceptor while F-111 was designed as attack aircraft from the start. And F-111 is larger and heavier.

@JanHudec Is that kind of like how the F-117A has an F at the beginning even though it's clearly not a fighter?  That would make some sense.  It looks like there have been a couple of Fighters with side by side though, (like the SU-34 seen below).  Uncommon, but happens.

@JayCarr, Su-34 is not really a fighter either. It has similar role to F-111. Here it shows that the term “figher” is really rather ambiguous. To be more precise, neither aircraft is “interceptor”.

Another naive guess: having pilots side by side may also increase roll momentum (and decrease maneuvrability)

@ManuH So, the assumption there being that for an intercepter, where roll rate really matters, they'd want to move the weight closer to the centerline, right?

You can call an airplane with side-by-side seating a fighter, but that doesn't make it one.  The pilot's visibility is badly damaged.

Side-by-side seating has been used in at least one night fighter (Skynight). Also the Mosquito could be considered a fighter or night fighter at least in some versions.  Of course, night fighters don't really need a great deal of maneuverability, or visibility (other than forwards), just performance.  Would be interesting to compile a list of ALL side-by-side fighters-- another ASE question?  But the question should spell out what counts as a fighter.

The tandem arrangement (one behind the other) is used on aircraft that have single seat variants. The F-15, F-16, and F/A-18 are all primarily single seat aircraft. It's much easier to stretch the nose out a bit to add a second position than to make it wider. This arrangement is also more aerodynamic, requiring less frontal area.
Fighter aircraft designed for a crew of two will often choose the tandem arrangement for aerodynamics. If both seats need forward visibility, there are compromises available. Some aircraft like the F-15E can be flown from both seats even though there is less visibility from the back. It's possible to have a bubble canopy in side-by-side configuration (this is particularly seen in aircraft kits), though it will create more drag than a tandem canopy.
Having the crew sit next to each other can be helpful for crew interaction and sharing instrumentation. The F-111 is one such special case:

The USAF wanted a tandem-seat aircraft for low-level penetration ground-attack, while the Navy wanted a shorter, high altitude interceptor with side-by-side seating to allow the pilot and RIO to share the radar display.

This may have also been easier to design the escape capsule. Of course the Navy changed their mind and went with the F-14 instead for better maneuverability.

Also, the F-111 was not really a fighter - more of a light bomber.

Also side by side allows for training situations, such as the ct114 tutor used in Canada. All fighter pilots must be trained in this jet first.

Well, some fighters (e.g. F-4 or F-14) did not have any single-seat version and still used tandem seating for aerodynamic reasons.

@SSumner - The F-111A, used by the USAF, was a light bomber in the vein of the F-105, but there were plans for an interceptor version. The radar systems of the F-14, and its AIM-54 Phoenix missile, were all originally designed to be carried by the F-111B naval interceptor, but when the Navy added in the ability to dogfight, the F-111 fell short of expectations and was passed over for a new design (the F-14)

Great point with crew interaction. One instance in which that took the priority over aerodynamics was the English Electric Lightning trainer variant. The new aircraft was unforgiving to learner pilots so it was designed to increase safety and facilitate learning.

@KeithS - that's my point. It made a poor fighter,  probably the Side by side seating didn't help.

@SSumner - "poor" is a strong word. It's not the F-111's fault that the U.S. top brass finally realized the lessons from Vietnam at just the wrong time. When the F-111 was conceived, the requirements were for a plane that got where it was going quickly, delivered ordinance, and got the hell out. The F-111 made as good a pure interceptor as it did a penetration bomber. It failed only once the Navy added dogfighting to the requirements (creating the modern "air superiority fighter") while the F-111B was in preproduction.

To be perfectly fair, the F-14 chosen instead was still a massive plane that, while it turned better than the F-111, could still have circles flown around it by light fighters in both Soviet and NATO arsenals, because the design still had to incorporate the interceptor role into this new profile of an "air superiority fighter". The F-15 got the same treatment as the USAF's direct answer to the MiG-25 interceptor. It wasn't until the high costs of both programs necessitated something cheaper to supply the bulk of both air fleets that the LWF competition produced the F-16 and F/A-18.

@KeithS - okay, yes, it made a poor air superiority fighter. That's more what I was going for

The main reason for having tandem seating in fighter aircrafts is that it offers better performance compared to the side by side version. This is why most of the high performance fighters have tandem seating arrangement.

"Sukhoi Su-30LL demonstrator flying along the runway at Zhangjiajie Hehua Airport less than 1 metre off the ground"by Xu Zheng - http://www.airliners.net/photo/Russia---Air/Sukhoi-Su-30LL/1025605/L/. Licensed under CC BY-SA 4.0 via Commons.
Another advantage in tandem seating arrangement is that conversion from single to double seat (and vice-versa) variant is relatively easy and requires minimal design change compared to the side by side configuration.

"Su-27 05" by Unknown - DefenseImagery.mil ID DD-ST-88-09314. Licensed under Public Domain via Commons.
The main advantage of the side-by-side seating arrangement is that it allows for better work sharing between the crew and reduces the need for duplication of flight instruments. The crew comfort is also (comparatively) better for long range flights, reducing fatigue.

"Sukhoi Su-34 at the MAKS-2013 (03)" by Doomych - Own work. Licensed under Public Domain via Commons.
Also, the customers (Air Force/Navy) may also decide which arrangement to use. For example, the initial postwar British trainers used side by side seating as RAF preferred this arrangement.
In case of B-52, the prototypes (and even the initial production aircraft) had a tandem seating arrangement.

"YB-52sideview" by Original uploader was Sf46 at en.wikipediaLater version(s) were uploaded by Nobunaga24 at en.wikipedia.(Original text: U.S. Air Force) - Transferred from en.wikipedia; Transfer was stated to be made by User:Nobunaga24.(Original text: USAF Museum Website). Licensed under Public Domain via Commons.
However, Gen. LeMay decided that a aside by side arrangement utilizes the copilot better and improved instrumentation and the arrangement was changed from tandem to side by side.

"Boeing B-52D-40-BW (SN 56-0695) and GAM-72 Quail decoy missile and trailer 061127-F-1234S-010" by US Goverment. Licensed under Public Domain via Commons.

(Photo #1) Sergi: Um, Igor, shouldn't the gear be down before we park?

@FreeMan This Sukhoi is still on the runway. Look at the Sukhoi parked on the tarmac in the background. This one's gear are down.

@ManuH - Actually, that first shot is of a famous Russian aerobatics pilot making a high-speed, low-altitude pass (less than one meter off the ground). He is actually too low in this shot to extend his gear.

One other point not mentioned here is that the F-111 was designed during the time frame that the Air Force and its contractors were experimenting with the lifting body principle. At that time, a lot of work was going into widening the fuselage and making it act like a wing. The main benefit the Air Force was looking for was that additional lift equals additional weapon payload. The contractors also explored that a lift-creating fuselage reduced structural stress on the wings and increased fuel capacity (increasing range).

As others have stated, tandem cockpits have advantages in that they create more slender bodies and require minimal modifications from single seat variants. The twin-seat modification is not always an issue, as many airplanes are designed from the get-go as two seaters, such as the F-4, F-14  and F-15. F-15 single and twin seaters share the same canopy, the second seat and instrumentation is all the difference between single and twin seaters (I'll leave the F-15E Strike Eagle aside, it has many modifications in the cockpit and in the rest of the aircraft).
F-15C and D have different canopies and minor airframe differences, see https://images-na.ssl-images-amazon.com/images/I/81Otq1glyTL._AC_SL1500_.jpg
Most tandem airplanes have different front and rear cockpits, the pilot and co-pilot/RIO/WSO have almost wholly independent tasks. Many rear seats don't even have flight controls; even if they do, visibility is very limited in the back so rear seat pilots take over flying only to reduce fwd pilot fatigue (in cruise or loiter) or when the fwd pilot is incapacitated.
OTOH, side-by-side designs improve cooperation between pilots and make for more spacious and less fatiguing cockpits. In the Su-34 and B-2, there is enough space between the seats for one occupant to lie down and sleep/rest, a huge luxury. In addition to long range missions, many aircraft with side-by-side cockpits have low-level penetration as a mission. Low-and-fast missions require excellent cooperation between pilots as they are seconds from hitting the ground at any given time. The F-111, B-1, Su-24 and Su-34 have side-by-side cockpits also for this reason.
The single seat Ka-50 helicopter was sidelined/superseded by the side-by-side Ka-52 mainly because of the excessive workload on the single pilot. Russians had the Ka-50-2 tandem cockpit design ready (concept for a Turkish Army tender, rejected in favor of ex-USMC Cobras), but they still went ahead with developing the Ka-52, ostensibly to improve pilot coordination. However, most attack helicopters (Mi-24, Cobra, Apache, Mi-28 and others) have tandem cockpits, to present a smaller frontal target to ground fire, in addition to the usual aerodynamic concern.

Welcome. Your answer could use some formatting to make it more readable. Please [edit] it and include some paragraph breaks. That's a lot of info with any sort of break at all.

Nice first answer. As @FreeMan suggested, you can add some pictures, though the description was clear for me, even without the pics.

@JetSonic I was just talking about formatting the text, not including pics, though pics are a bonus.

Duh, it's my habit of using Shift+Enter instead of Enter. Thanks all for the heads up.